In conventional manner, an inorganic membrane is constituted by a porous support made mainly in the form of a tube whose inside surface is provided with at least one separating layer whose nature and shape are adapted to separate molecules or particles contained in the liquid medium flowing inside the tube. Inorganic membranes have the special feature of possessing high degrees of mechanical strength and of presenting thermal and chemical stability. Inorganic membranes thus exhibit stability and performance that are considerably better than those of the other category of membranes, namely organic membranes.
The separator layers are made by depositing a suspension on the support, the suspension including various components generally in the form of grains. The thickness of the deposit is controlled by the parameters of suspension concentration and of time of contact between the suspension and the support. The assembly is subjected to drying so that the grains come closer together, while the liquid in which they were suspended is removed by vapor pressure. To consolidate the deposit, the membrane is subjected to a baking operation.
In simplified terms, the deposit of a separator layer can be considered as a pile of spheres separated by gaps representing the initial porosity of the separator layer. The mean equivalent diameter of the pores is thus dependent on the size of the spheres.
In known manner, a technique has been proposed for depositing separator layers enabling frusto-conical pore shapes to be obtained, with the smaller diameter being in contact with the fluid to be treated. That technique makes it possible to make a membrane that is thin, so as to obtain a high transit speed for components that are not stopped by the membrane.
For inorganic membranes, the making of a conically-shaped pore requires a plurality of steps. Each of those steps comprises depositing a layer defined by its thickness and by the mean equivalent diameter of its pores. The layers are superposed on one another, each time reducing the thickness and the equivalent mean diameter of the pores. Membranes of that type are commonly referred to as "composite". By way of example, an inorganic microfiltration membrane having a separation power of 0.2 .mu.m corresponding to the diameter of the smallest particle that cannot pass through the membrane may comprise:
a "support" layer that is porous and that provides mechanical strength, having a thickness of 2 mm and pores of a mean equivalent diameter of 5 .mu.m to 6 .mu.m; PA1 a layer having a thickness of 80 .mu.m and pores of a mean equivalent diameter of 1.5 .mu.m; and PA1 a layer having a thickness of 50 .mu.m and pores of a mean equivalent diameter of 0.2 .mu.m. PA1 either during production of the powder by reaction in the gaseous, liquid, or solid state. All three states are possible since they depend on the method of synthesizing the aggregate. PA1 or else, by solid state reaction between individual particles that have already been formed. Particles are subjected to heat treatment, thereby considerably reducing particle viscosity and enabling matter to be interchanged with neighboring particles. After cooling, the aggregate made in this way is either in amorphous form or else in crystalline form. In either case, some quantity of material exists between the original individual particles. PA1 in shaping and baking a porous substrate; and PA1 in depositing on the substrate a suspension containing at least one sinterable composition designed to constitute a filter layer after baking. PA1 in selecting, as the sinterable composition, a powder including at least aggregates made up of individual particles; and PA1 in destroying the aggregates contained in the sinterable composition in such a manner as to cause the composition to be composed essentially of individual particles.
In practice, deposition of each layer must be terminated before moving onto the next. As a result, fabrication of an inorganic membrane is therefore relatively lengthy. Furthermore, multiplying the number of layers in order to make it possible for the pores to tend towards a conical shape, also makes it possible to reduce penetration of one layer into the layer beneath. For example, a layer having pores of a mean equivalent diameter of 5 nm can be obtained from a suspension having grains with a mean diameter of 16.5 nm. If the deposit is performed on a layer of 6 .mu.m, of 1.5 .mu.m, or of 0.2 .mu.m, then the grains penetrate fully into the pores of said layer. It is therefore difficult or even impossible to form such a deposit. Conversely, if the deposit is performed on a 50 nanometer layer, then there is little penetration and the deposit can be made.
Nevertheless, interpenetration of the layers does exist and that has an effect on the real diffusion area of the membrane and consequently on the speed with which fluid can pass through the filter element. As explained above, multiplying the number of layers reduces such penetration. Unfortunately, increasing the number of layers gives rise to a fabrication method that is relatively lengthy and expensive and reduces the transit speed. It therefore appears necessary to reduce the number of layers and to prevent them interpenetrating so as to obtain simultaneously low headloss and a real filter area that is close to the geometrical area of the filter element.
Proposals have been made in patent application FR 2 502 508 to achieve the above objects by using a material that can be eliminated thermally for the purpose of plugging the pores in the coarse layer.
Such a method suffers from the major drawback due to the fact that the technique requires prior deposition of the material that can be eliminated thermally.
Another method is described in patent U.S. Pat. No. 3,977,967 which seeks to associate a size of aggregate with the mean diameter of the pores in the substrate.
The Applicant has developed another technique that seeks to reduce the number of separation layers and to avoid them interpenetrating while still obtaining a real filter area that is close to the geometrical area of the filter element, and while also obtaining low headloss. The present invention enables this object to be achieved by implementing the suspensions used for depositing the separator layers in a special manner.
To understand the invention properly, it is essential to define accurately the following terms which are used below in the description.
Particle: a quantity of material corresponding to a monocrystal. If the material is amorphous, the particle represents the quantity of material that will give rise to a monocrystal after heat treatment.
Aggregate: An aggregate is made up of individual particles. Bonding between the individual particles is of the chemical type and it is obtained:
With vapor phase reactions, synthesis takes place while the gas is cooling, i.e. the gas transforms into solid nuclei. If the nuclei remain individualized, then individual particles are obtained. Very frequently, the nuclei coalesce to give rise to an aggregate. Within such an aggregate, there exists some quantity of material between the various nuclei.
For liquid medium reactions, the product formed is generally an amorphous precipitate. The precipitate is in the form of a microporous solid having large specific area and can be represented as an assembly of individual particles.
For solid medium reactions, the size of the products formed depends on the distance between the metal atoms within the precursor molecule of the solid medium.
To sum, an aggregate may be defined as an assembly of individual particles such that there exists continuity of particle matter between particles that are adjacent within the aggregate.
Cluster: clusters are formed in two different ways:
a) Chemically by the action of inorganic salts or of ionic polyelectrolytes that encourage particles to assemble together.
Cluster formation depends on the interactions that are possible between the particles. Each particle in suspension carries electrical charge on its surface that is the result either of small imperfections in its crystal lattice or else of adsorbing ions. The electrically charged surface then attracts ions of opposite sign to attempt to achieve an electrically neutral state. As a result, a layer of liquid that is enriched in ions of sign opposite to the charge on the particle collects around the solid and the resulting charge depends on the nature and the concentration of ions in the liquid in suspension. There thus exists an ion concentration for which the resulting charge of the particle is zero (the isoelectric point). Within a suspension, all of the particles have identical charge. The forces between the particles are thus mainly repulsive. It is necessary to add reagents capable of neutralizing surface charge so as to achieve an electrically neutral state. Under such conditions, the effectiveness of attractive forces is important. The various attractive forces are as follows:
Van Der Waals forces due to interactions between molecules or atoms;
surface tension forces; and
chemical forces due to chemical bonds between particles.
Van Der Waals forces are interactions between the atoms and the molecules of the particles. They depend on crystal properties and dissolved ions have little influence on their magnitude. Surface tension forces appear only in a liquid mixture. Chemical forces originate from hydrogen or covalent type bonds.
b) Physically where the particles are fixed by chemical bridges on molecules that are very long.
Macromolecules having a molar mass of several million Daltons have real backbone lengths that are much longer than those of individual particles. In addition, it is possible to determine solvation conditions for such molecules so that their backbones are fully stretched and so that there exist chemical groups all along the backbone capable of attaching particles. Under such conditions, a molecule backbone is capable of collecting a plurality of individual particles to form a cluster. Bonding between the backbone and a particle is purely chemical and is of the hydrogen or covalent type. In spite of the bonding being of the chemical type, this method of forming clusters is known as the "physical" method.
To sum up, a cluster is an assembly of particles in which there is no continuity of particle matter between adjacent particles within the cluster.
Layer: the purpose of a layer is to perform separation in a population of molecules or particles. It is defined as a continuous stack of particles and/or aggregates and/or clusters. The voids inside the stack constitute the porosity of the layer. The system whereby porosity is measured treats such voids as stacks of cylinders that are mutually parallel. Pore diameter thus represents the diameter of the cylinders that are equivalent to the porosity.
Porosity: the volume of the voids within a stack. It is characterized as the ratio of void volume divided by total volume.
Equivalent pore diameter: the diameter of the cylinders equivalent to the porosity.
In the majority of cases, suspensions used for depositing separator layers are made up of mixtures of particles, aggregates, and clusters. The Applicant has the merit of showing that the porosity of the layer depends on the nature of the components from which the separator layers are built up. Thus, it has been observed that a deposit made of aggregates, in particular when made by the technique of patent U.S. Pat. No. 3,977,967, gives rise to a pore distribution that is non-uniform and that is spread over a wide range because of the combination of porosity between aggregates and also porosity within aggregates. In contrast, a pore distribution that is uniform and that has a narrow spread can be obtained when making a deposit from particles.